Concrete screw spinning without biting: bit, hole and driver

“The screw goes in, then at the end of the run it spins and won't tighten any more.” When you tell me that, there are three causes, always the same: a worn or wrong drill bit, a dirty hole, an impact driver that is too aggressive. The screw is usually innocent.

How a concrete screw works

It does not expand: the hardened thread cuts its own thread into the concrete as it goes in. That is why it is fitted without a plug, comes out without leaving anything sticking out, and can be fixed close to other holes. But precisely because it cuts, the relationship between thread and hole has to be the one specified.

The drill bit is the first thing to check

The bit diameter is the one stated by the manufacturer for that screw, not “one size up to be safe”. An oversize hole leaves the thread little material to bite into, and the screw strips. And a worn bit makes a narrower hole: the screw jams halfway or snaps. In the collection you will find DeWalt Blue-Tip, Fischer UltraCut FBS II, SPIT Tapcon and Bossong CLS: each one has its own table, and it needs to be read.

Depth and hole cleaning

The hole must be drilled deeper than the embedment length, to leave room for the dust and swarf the screw pushes ahead of it. If the hole is exact to the millimetre, the screw reaches the bottom, meets the compacted dust and goes no further. A blow-out before driving always helps, and in critical applications the cleaning is whatever the data sheet prescribes.

Impact driver: use it with judgement

Many concrete screws are driven with an impact driver, but once the head seats, you stop. Carrying on hammering breaks the thread just formed in the concrete and the screw spins freely. On important fixings the final torque is checked with a torque wrench, to the value in the technical data sheet.

And a screw that has stripped is not put back into the same hole in the hope that it will hold: you change position or move to a diameter specified for that case.

Substrate, edges and loads

Minimum edge distances and spacing between fixings are not pedantry: near an edge the concrete breaks away in a cone. Cracked concrete, seismic loads or safety-critical loads call for a product with a European Technical Assessment (ETA) for that application. For high or dynamic loads, the same collection includes Fischer mechanical anchors, expansion and undercut types: sometimes they are the best choice.

On solid brick, hollow brick or aerated concrete the picture changes, and you need screws or anchors designed for those materials.

What I need to know

Tell me the substrate (concrete, and whether it is old or cracked), what you are fixing and its load, the thickness of the part to be fixed, whether you are near an edge and whether it is outdoors, where suitable galvanising or stainless steel is needed. I will suggest the screw or anchor, length and bit, so that the first hole is also the last.

We stock these

A few items from the range we keep. If you cannot find the size or the model you need, just ask: what you see online is only part of what we can source for you.

Concrete screw Tapcon5Xtrem Hfl SPIT -326459SPIT Concrete screw Tapcon5Xtrem Rod SPIT -326457SPIT Concrete screw Tapcon5Xtrem Dome SPIT -326454SPIT Concrete screw Tapcon Xtrem Lt Hfl A4 SPIT -302994SPIT Concrete screw Tapcon Xtrem Lt Csk A4 SPIT -302993SPIT Concrete screw Anchor Tapcon Plus Hfl SPIT -176339SPIT

Browse the whole category: Concrete Screws (Screw Anchors)